2. Where did our diseases come from?
Africa: homeland of mankind and malaria
Africa is the ancestral homeland of mankind. Our species originated there perhaps 100,000 years ago. From Africa, humans spread through the Middle East and around the world. Not surprisingly, most of our original diseases evolved alongside (or inside) their human hosts in Africa, so the human race grew up in constant contact with parasites such as trypanosomes, which cause sleeping sickness, and Plasmodium, which causes malaria.
Many diseases adapted to tropical conditions were left behind by those who migrated to colder regions. In particular, many parasites that need a warm climate failed to adapt to the temperate zones. Conditions inside the human body remain fairly constant. Consequently, the susceptibility of a disease agent to climate depends on how much time it spends outside the body between infections. Bacteria and viruses that are passed directly from person to person are affected little. Parasitic worms whose larvae develop in rivers or lakes before reinfecting human hosts are highly susceptible. Diseases that rely on insects to spread them are greatly affected by climate because their insect vectors often cannot survive colder winters. Thus, mankind left behind malaria, sleeping sickness, yellow fever, and many other insect-borne diseases when we emerged from the tropics.
These diseases are still a major problem in tropical regions. According to the World Health Organization (WHO), some 500 million clinically observed cases of malaria cause a little over a million deaths each year, the majority in Africa. Of these deaths, about half are of children younger than five years old. Recently, AIDS overtook malaria and tuberculosis to become the leading cause of death among the infectious diseases, with around three million deaths. (Diarrheal disease and respiratory infections still head the mortality tables, but these are each due to several different infectious agents.) Tuberculosis kills around 1.5 million victims per year, slightly more than malaria. However, these deaths result from about 10 million cases, far fewer than the 500 million cases of malaria. Relatively few malaria victims die outright. Instead, they suffer life-long debilitation, which not only lowers their productivity, but also makes them vulnerable to other infections.
How important was malaria?
Malaria is sometimes quoted as having killed more people than all the wars and all the plagues in recorded human history. But although infectious disease as a whole has undoubtedly killed far more people than warfare, little compelling evidence indicates that malaria has outperformed all other infectious diseases. Although malaria has taken a steady death toll in Africa and other tropical zones, it was absent on the American continent until European colonization. Furthermore, although malaria nibbled at the heels of Europe until recently by infesting marshlands, the dense urban populations of temperate Europe and Asia were fairly unaffected. Moreover, until the recent population explosion, the population density of Africa and other tropical regions where malaria is endemic was relatively sparse.
Even in Africa, the evolutionary homeland of both man and his earliest diseases, was malaria really the number one killer before the last few centuries? Today Plasmodium falciparum causes most lethal attacks of malaria, whereas the other three species of malaria cause relatively milder disease and are rarely lethal. Although P. falciparum is presently spreading from Africa around the tropical world, the sickle cell mutation that provides resistance is found only in Africans indigenous to regions harboring P. falciparum malaria. The fact that the sickle cell trait is so harmful by itself suggests that it is a recent, emergency, evolutionary adaptation. Over longer periods, we would expect the build-up of resistance with less deleterious side effects, as is the case for many other diseases, including the milder variants of malaria. This suggests that the malignant, falciparum form of malaria is of relatively recent origin and that, even in Africa, malaria was present in its milder forms for much of early human history.
Moreover, in precolonial Africa, many other diseases that have since been largely eradicated due to Western technology were still active. Relative to malaria, yellow fever may be trivial today, but in the early colonial period, sailors to tropical parts feared it at least as much. Again, among many West African tribes, smallpox, which has now been completely eradicated, was historically feared the most. Although malaria has survived the onslaught of modern technology better, this does not necessarily mean it was the major killer before other diseases were brought under control.
Our fellow travelers
Malaria is the best known example of a disease that has accompanied our species from its earliest beginnings and remains a major health problem. However, it is by no means the only disease to have accompanied us since our origin. Tuberculosis, herpes, and typhoid are other well-known examples. This raises the issue of how an infectious disease avoids getting left behind when its human victims consist only of small, scattered bands.
Consider first a “recent” disease such as measles. This humans-only disease is highly contagious and is spread from human to human without relying on any insects or other carriers. After recovery, humans gain immunity from measles. Consequently, measles faces the problem of constantly finding fresh victims. When measles has finished infecting all members of a small isolated tribe, it has nowhere to go. Thus, diseases such as measles cannot persist unless civilization provides a densely packed crowd of victims. Clearly, measles is not one of our original diseases; we consider its origins later.
One way for a disease to avoid the predicament of measles is to be shared among multiple animals. Malaria and sleeping sickness are examples of this approach. Another approach is to remain dormant inside a host until fresh victims are available. Herpes, caused by a virus, and typhoid and tuberculosis, caused by bacteria, have all taken this route. Viruses of the herpes family may lie quiescent in nerve cells for years until provoked by stress to emerge. They may then spread to new victims. Typhoid can hide in the gall bladder of human hosts who show no symptoms but are a constant source of infection to others. Tuberculosis, caused by Mycobacterium tuberculosis, hibernates in the lungs.
Human remains showing signs characteristic of tuberculosis (TB) have been found dating as early as the Neolithic period, when settled agricultural communities first appeared (9,000 B.C. onward). X-rays of Peruvian mummies dating to before the European conquest show signs of tuberculosis, implying that the American Indians brought TB with them when they crossed the Bering Straits some 10,000–15,000 years ago. Extraction of DNA characteristic of Mycobacterium tuberculosis from some of these mummies has confirmed that it really was tuberculosis. The signs of TB in the skull of a half-million-year-old Homo erectus from Turkey are vastly more ancient.
It was once thought that tuberculosis might have moved into the human population from cattle, which suffer from a closely related form of the disease. However, recent DNA analysis suggests the reverse—that we humans transmitted tuberculosis to cows after domestication.
Many human diseases originated in animals
Despite the new DNA evidence that exonerates the cow from spreading tuberculosis, most of our present infections probably did originate from other animals. It seems likely that prehistoric hunter-gatherers were relatively free of infectious diseases, compared with historical and present-day man. The unusual susceptibility of American Indians to most diseases brought across the Atlantic from the Old World argues that the indigenous people of the American continent had never been exposed to these diseases. This implies that these diseases emerged after the ancestral American Indians split off from their Asian relatives approximately 15,000 years ago. Because the migrating tribes evidently did not import them into America, it seems that smallpox, measles, and so forth must have been human diseases for less than 15,000 years—perhaps less, even, than that.
Before rushing forward, a word of caution is in order. We are fairly sure that malaria is an ancient disease. However, malaria was not present among American Indians before contact with the Old World was reestablished. The reason is that malaria is carried by mosquitoes, which failed to survive when humans migrated from the tropics into the colder regions of Asia. The Asian ancestors of the American Indians had therefore left malaria behind before they entered the American continent. When invoking New World susceptibility for the age of a disease, we must keep this factor in mind. Other tropical diseases that cannot persist in colder climates may also be ancient despite not being carried to the Americas.
Note also that, apart from the domestic dog, the American Indians lacked the domestic animals characteristic of the Old World. When the Bering Strait was crossed, cattle, sheep, horses, and pigs had not yet been domesticated by the tribes who made the crossing. Many human diseases have come from these animals. Because the first humans to colonize the Americas did not take these animals along, they could not have caught their diseases after migrating.
As humans expanded around the globe and populations grew ever denser, our species became a living paradise for infectious disease. No other large animal in the known history of our planet has provided such crowds of individuals, packed closely together, just waiting for some pestilence to move in and multiply. Over the ages, infectious diseases have migrated from their original hosts whenever they made contact with the human species. Animals that have the closest relationships with humans have been the source of most diseases. This includes not just the deliberately domesticated animals, but also the rats and mice that have taken up residence in and around human settlements. Even today, most human towns and cities are home to more rodents than humans.
Dense herds of domestic animals and fields of closely planted crops have provided similar opportunities for colonization by diseases from related wild animals and plants. Some of these, in turn, have moved on to humans. Today, as the remaining jungles and rain forests are being explored and exploited, they have yielded more novel diseases. Lassa fever, Hantavirus, and Ebolavirus have all made the jump to man, the ubiquitous host. Although mankind has done a good job of exterminating other larger animals, we have kept many of their diseases.
Recent diseases from animals
When Hippocrates compiled his treatise in the fifth century B.C., the ancient Greeks did not know of smallpox, measles, bubonic plague (an Asian disease), syphilis (an American import), yellow fever (from Africa), or leprosy. They were aware of herpes, typhoid and/or typhus, tetanus, amebic dysentery, rheumatic fever, chlamydia (both venereal and trachoma), and gonorrhea (or something very similar).
Zhouhou Beijifang, written by Ge Hong in fourth-century-A.D. China, lists malaria, erysipelas, typhoid, dysentery, and cholera. In contrast to the ancient Greeks, leprosy and smallpox were now known, implying that these appeared roughly 2,000 years ago. These records, together with a variety of other ancient accounts, suggest that many diseases we are familiar with today were absent in early historical times and have appeared only within the last thousand years or so.
Viral diseases, which colonized humans only after civilization provided sufficiently crowded victims, include mumps, measles, German measles, smallpox, polio, influenza, and even the common cold. At least our hunter-gatherer forefathers didn’t have to worry about scaring away the game with violent sneezing! These viral diseases all have close relatives in various animals. After making the jump to humans, they adapted over the centuries to their new hosts and, in many cases, lost the ability to infect their original hosts. For several of these diseases, evidence suggests that they were originally more virulent and have become milder over the years.
For all these diseases, victims who recover become immune. Consequently, these viruses must all keep moving through a constant supply of new hosts. Flu and colds do return year after year and reinfect the same people, but each successive epidemic comes from a newly evolved strain of virus. Although you may be reinfected with such new strains, you remain permanently immune to variants of flu or cold viruses from previous years.
Smallpox is a good candidate for a very recent addition to humanity’s panorama of parasites. It was still quite virulent in the twentieth century, even among Old World populations. Smallpox was once thought to be derived from cowpox. However, recent genetic analysis has shown that camelpox is its nearest relative, so transfer from camels to humans in the Middle East seems plausible.
Probably the earliest recorded smallpox epidemic is mentioned in the Koran, which is consistent with a Middle Eastern origin. The siege of Mecca by the Ethiopians in 569/570 A.D. was routed by this epidemic. A.D. 570 was the year Mohammed was born, and Islam had not yet been founded. Nevertheless, Mecca was already the holy city of the Arabs, and the Ethiopian Christians were hoping to destroy Allah’s sacred shrine, the Ka’aba. The Koran credits Allah with slaughtering the Ethiopians.
The Islamic expansion of the seventh and eighth centuries spread smallpox throughout the Mediterranean area—or perhaps we might better say that smallpox cleared the way for Islam to expand, much as it later cleared the way for the Spanish conquest of Central America. The Islamic Empire crossed the Straits of Gibraltar from North Africa to conquer Spain in 710. The Arabs were still in Spain when the Black Death pandemic of the mid-1300s occurred. This seems to have helped tip the situation in favor of the native Spanish kingdoms, and the Arabs were gradually expelled over the next couple centuries. Smallpox moved quickly. In the year 737 A.D., a great smallpox epidemic in Japan caused major depopulation.
Which diseases from which animals?
Although apes and monkeys are more closely related to humans, they have provided few diseases. Herded livestock and rodent pests are more frequent sources of human infections. This is not really surprising: The greater the population density of the animals a disease infects, the more opportunity that disease has to grow more virulent, to evolve new variants, and to spread. In addition to population density, another critical factor is intimacy. Cattle, sheep, and goats are grazing animals and live in fields separate from their human owners. In contrast, pigs and chickens are found in farmyards and have had much closer contact with humans. Dogs live closest of all, often sharing the house with humans and the uninvited mice and rats found in and around all human habitations. As a result, dogs, pigs, chickens, and mice have tended to pass on more infections than sheep, goats, and cattle.
Nonetheless, our cousins the apes and monkeys have provided us with a few infections. Amebic dysentery probably came from the Rhesus monkey, which lives in the forests of Asia. AIDS, from the Human Immunodeficiency Virus (HIV), comes from African monkeys via the chimpanzee. Some 30%–50% of green monkeys found in Africa today carry Simian Immunodeficiency Virus (SIV), a close relative of HIV. In contrast, green monkeys living in the Caribbean show no traces of infection. These monkeys were brought from Africa during the seventeenth and eighteenth centuries, indicating that the modern group of SIV/HIV viruses has emerged since then.
Not all human diseases come from other mammals. Birds are almost certainly responsible for influenza, which still circulates among pigs, people, chickens, and ducks. Even today, birds live in large flocks, and in earlier times, colossal swarms of waterfowl inhabited the wetlands of Europe and North America. As the herds and flocks of domestic animals grew in size, they picked up diseases from birds and eventually passed them on.
Who owns which disease?
Despite our self-centered outlook, very few diseases are restricted to humans alone. If no reservoir of infection exists among animals, curing, immunizing, or quarantining all human victims should drive such a disease to extinction. This has been done for smallpox. The WHO began the eradication program in 1966 and completed it in the 1970s. So far, smallpox is the only human disease to be totally eradicated in the wild, although ongoing attempts have targeted polio and the parasitic Guinea worm.
Total eradication is rarely possible because most diseases, even those that primarily infect humans, also infect other animals. Malaria, yellow fever, and many tropical diseases also infect monkeys and apes; bubonic plague and rabies can infect cats and dogs. Are malaria and rabies really human diseases, or are they animal diseases that humans sometimes have the misfortune to catch? Although the dividing line is somewhat arbitrary, we can distinguish diseases that circulate mostly among humans, animal diseases that are occasionally caught by humans, and shared diseases that routinely infect several host species.
Some animal infections are occasionally transmitted to humans by accident and are rarely, if ever, passed from person to person. Rabies is certainly infectious but is almost never acquired from being bitten by another person. Glanders is a disease of horses that is occasionally transmitted to humans, such as stable hands who come in close contact with infected animals. Anthrax is primarily a disease of cattle but may cause epidemics with high mortality among humans. Under natural conditions—that is, before civilization—these diseases were probably unknown among humans. Only after the domestication of cattle, horses, and dogs did their diseases come into close enough contact to jump the species boundary. Some of these animal diseases adapted to their human hosts and have become genuine human diseases. Others have remained primarily animal diseases and only sporadically infect humans.
Different diseases evolve at different rates. Generally, the fewer genes are involved, the more rapidly the diseases evolve. Thus, viruses evolve faster than bacteria, which, in turn, evolve faster than protozoa. Consequently, many virus diseases have evolved so quickly that distinct human diseases have appeared. Often these have diverged so far from their ancestors that they no longer infect even other animals. Good examples are smallpox, probably derived from camelpox, and measles, which is related to distemper, a disease of dogs and related carnivores. In contrast, bacteria evolve more slowly than viruses, and we still share most of our bacterial diseases with other animals, although there are often specialized, human-adapted variants. For example, “epidemic” typhus is a human-specific version of “murine” typhus, which infects both rodents and humans. Protozoa evolve more slowly than bacteria, and we still share our best-known protozoan disease, malaria, with other apes and monkeys.
Are new diseases virulent to start with?
We often get the impression that whenever a novel disease jumps the species boundary and infects humans for the first time, it is incredibly virulent, as with Lassa fever or Ebolavirus. This is an artifact of journalism. If half a dozen people in some out-of-the-way place contracted a novel but mild illness, it would probably not even be investigated, let alone hit the headlines. Microbes are constantly invading the human body. Few make it past the human defense system. If a novel infectious agent does survive, whether it causes a mild or lethal illness is largely a matter of chance.
Lassa and Ebola illustrate this well. Recent investigations have revealed that milder strains of Lassa fever and Ebolaviruses have been infecting humans many years before the official “discoveries.” People who are infected with a virus are immune to further infection, and their blood contains circulating antibodies that are specific to the virus in question. Analysis of blood samples from inhabitants close to the Lassa River in Nigeria revealed frequent cases of people with antibodies to Lassa fever virus. Many had no recollection of any illness recognizable as Lassa fever; others remembered attacks of moderate to severe fever, often attributed to malaria. Earlier outbreaks of Lassa fever in Nigeria thus ranged in virulence from scarcely noticeable to moderate, with the latter usually diagnosed as “aberrant” cases of malaria. Lassa was recognized as a new disease only when a highly virulent version hit the headlines in 1969. Ebolavirus has behaved in much the same way, emerging officially in Zaire in 1976.
Over the long term, novel diseases may adapt to humans or may go extinct. Adaptation does not imply that the disease becomes mild—merely that it gains the ability to survive and multiply in humans. The level of virulence acquired depends on the mode of transmission and how plentiful, crowded, and unhygienic the human hosts are.
Measles and its relatives
Among Old World populations, measles is a relatively mild childhood disease, only rarely causing severe complications. It is transmitted from person to person, and once a person has measles, he becomes immune and the virus disappears from his body. For measles virus to stay in circulation, it must constantly find a new supply of victims to infect. In societies where essentially everyone catches, survives, and becomes immune to measles as children, the only new hosts for measles are newborn children.
Calculations indicate that a human population of 300,000–500,000 individuals in frequent contact with each other is necessary to provide new children at a sufficient rate to prevent measles from going extinct. Before approximately 1,000–500 B.C., there were no individual cities with populations over a quarter of a million. The first to appear were Babylon, capital of the Babylonian Empire, and Ninevah, capital of the Assyrian Empire, followed shortly by Europe’s first real city, Athens. Until these were available, measles could not have been easily maintained as a human disease.
Before this period, diseases such as measles had to rely on moving from town to village within a region of civilization linked by road or river. Dense enough populations were found in the Middle East, in the valleys of the Tigris and Euphrates rivers (Mesopotamia), starting around 3,000 B.C., and later in the valleys of the Nile (Egypt), the Indus (North India and Pakistan), and the Yellow River (Northern China). Each of these areas had enough people to keep an epidemic disease in circulation, as long as the separate communities were in efficient contact. This varied considerably over the centuries, and no doubt the spread of infectious disease fluctuated wildly as a result. Many diseases probably jumped from animals to man during this early period, burned their way through a few unfortunate towns in close contact, but then failed to spread any further. Some animal diseases may have jumped several times before becoming established in the human population.
Where did measles come from? Probably from man’s best friend, the dog. Measles is a member of the distemper virus family, the Morbilliviruses. Distemper viruses are typically found in carnivores and include seal distemper and dolphin distemper, in addition to canine distemper. Canine distemper can also infect other carnivores and is found among sea lions and hyenas. Humans probably caught measles from dogs a couple thousand years ago, and it has since evolved into a milder form.
Comparing DNA sequences shows that rinderpest of cattle is the closest present-day relative of measles. However, rinderpest is an extremely virulent disease that still causes mass mortality among cattle and related wild animals (buffalo, antelopes, giraffes, wildebeest). Viruses that spread directly from person to person, or between individuals of closely related species, usually evolve to lower virulence over time (as you will see in Chapter 3, “Transmission, Overcrowding, and Virulence”). Taking this into account, the most likely scenario is that dogs passed distemper to humans, where it evolved into measles. More recently, humans passed the virus to cattle, where it evolved into rinderpest but has not yet had time to drop significantly in virulence. In agreement with this scenario, the major rinderpest outbreaks in the 1990s showed lower death rates in domestic cattle but still caused 50% or more mortality in buffalo, kudu, and other game animals. This argues that rinderpest has partly adapted to domestic cattle and has spread from there to wild game animals, where it is still highly virulent.
Measles in ancient history
Can we pinpoint the origin of measles more accurately? Not really, but we can amuse ourselves guessing. In the Iliad, Homer describes how Apollo, who favored the Trojans, sent a plague on the Greek army besieging Troy. Although Apollo was the archer god, he was not the god of war; his arrows carried pestilence. He was also the god of medicine—what gods send, they can also remove, if asked politely.
Then he sat down apart from the ships and let fly an arrow: terrible was the twang of the silver bow. The mules he assailed first and the swift dogs, but then on the men themselves he let fly his stinging shafts, and struck; and constantly the pyres of the dead burned thick.
The Iliad was written around 700–800 B.C., several hundred years after the actual siege of Troy. In his commentary written some 400 years later still, Aristotle was puzzled why Apollo would attack the mules first. Knowing as we do today that canine distemper is closely related to both measles and rinderpest, we might argue that Homer reported what actually happened: an outbreak of an early distemper virus still shared by dogs, mules, and humans. If so, the Greeks probably caught it from the more densely populated city of Troy.
As human population density increased over the next few centuries, a strain of this shared morbillivirus may have adapted solely to humans. Voilà, measles! It is even conceivable that the Great Plague of Athens (430 B.C.) was caused by the ancestral, highly virulent form of measles. At that time, Athens had just become the only region of Europe densely populated enough to maintain a disease such as measles. (At this time, the city of Athens is estimated to have had around 250,000 people and Attica, the surrounding area, another 200,000 or so.)
The morbilliviruses include several other mild diseases, including mumps and the human parainfluenza viruses. As you might guess, parainfluenza viruses cause symptoms similar to but less severe than genuine influenza. Close relatives of these are known in birds, pigs, and monkeys. Take your choice. Successive waves of infection by a series of related viruses probably entered the human population from various animals. Those that were originally lethal have by now become milder, and their symptoms are so vague that only mumps still merits a name of its own.
Diseases from rodents
Rats and mice bear the same relationship to sheep and cows as weeds bear to wheat and maize. They are neither truly wild nor truly domestic. Even though we never intended to domesticate them, our rodent pests depend as much on humans for their homes and food as animals we have deliberately tamed. Rats live in attics, basements, sewers, storm drains, and any other underground tunnels. House mice live in cavities within walls, and field mice live in barns, haystacks, and cornfields. Rodents scavenge leftovers and steal grain, cheese, and other stored foods. Though rarely seen, rats usually outnumber people in a typical human city.
The mouse was one of the earliest domestic animals. The Natufian culture, found in the Middle East about 12,000 years ago, was transitional between hunter-gatherer and a settled agricultural way of life. The remains of domestic mice were found when excavating the Natufian layer at Jericho. Presumably, mice were attracted to the stored grain and moved in with the humans. Today rats and mice star in cartoon movies, and the most famous American of all time is a mouse whose only close competitors are a duck and a rabbit. In historical times, rodents were more often star performers in spreading pestilence.
Some diseases are spread directly by rodents, but more often insects act as intermediaries. The most famous example is the Black Death (bubonic plague), spread by fleas carried by rats. This is not a uniquely human disease, but is shared by many animals.
Typhus fever is carried by lice or fleas. Epidemic typhus has adapted for human-to-human transmission via the human body louse. It is closely related to murine typhus, which spreads among rodents or from rodents to humans via fleas. The rodent disease is the ancestor of the more specialized and more virulent epidemic typhus. This specialized human version of typhus has been responsible for massive casualties among soldiers during military campaigns and among the inmates of prisons and other institutions. Most of Napoleon’s casualties during his disastrous Russian campaign fell prey to typhus. Where lice are free to roam over blankets, clothes, and hair, typhus is not long in following.
Most diseases spread by rats and mice are actually carried by the fleas, lice, or ticks that live on the rodents. However, several cases of direct transmission are noteworthy. Some of the emerging viral diseases, such as Hantavirus and Lassa fever, are spread by contact with rodent droppings or urine. These are both primarily diseases of wild rodents, not house mice or city rats. Unless they establish themselves in the domestic rodent population, they are unlikely to become a serious health problem. For now, as with Ebolavirus, they are a scary but rare threat, lurking on the fringes of civilization.
Leprosy is a relatively new disease
Most people are under the impression that tuberculosis and leprosy, both the result of infection by Mycobacteria, are ancient scourges of the human race. This may be true for tuberculosis, but leprosy is a much younger disease and probably came from mice. Both diseases are spread by direct personal contact, so both are under evolutionary pressure to adapt by becoming milder. Ancient traditions depict leprosy as highly contagious and virulent, hence the need to segregate lepers. In apparent disagreement with this, leprosy seems rather difficult to catch nowadays, even with prolonged exposure. Only about 10% of the family members of a leprosy victim catch the disease, despite being in close contact and also being genetically related.
Before dismissing the earlier views, let’s consider them from an evolutionary perspective. Leprosy probably was more contagious in the past. Over time, many people sensitive to leprosy have undoubtedly died, so surviving humans are inherently more resistant. In addition, to promote its own distribution, the leprosy bacterium has probably become less virulent. Although leprosy is eventually fatal if untreated, this takes a long time, thus giving the bacteria longer to find a new host.
The disease known today as leprosy is caused by Mycobacterium leprae. It is sometimes called Hansen’s disease, not merely to reduce the social stigma to its victims, but because assorted diseases causing skin lesions and disfigurement were lumped together with leprosy until fairly recently. True leprosy causes characteristic bone loss at the extremities and can be diagnosed unambiguously in skeletons dug up by archeologists. The earliest traces of leprosy found so far date to the second century B.C. in skeletons from an oasis in the Sinai desert. Thus, the leprosy mentioned in the Old Testament is not the same as our modern disease.
Where did modern leprosy come from? The culprit is probably the mouse, which harbors a severe disease caused by the related bacterium, Mycobacterium lepraemurium. Cats also suffer from a form of leprosy, although the disease rarely penetrates the internal organs and the skin lesions generally clear up naturally after a few months. Although mice get a much nastier disease, both cat and mouse leprosy are caused by the same bacterium. We may guess that around 200 B.C., M. lepraemurium made the jump into humans and then gradually degenerated into Mycobacterium leprae, which can no longer grow outside a living victim or reinfect mice.
What goes around comes around
Many human diseases, both infectious and noninfectious, are self-inflicted wounds. The present prevalence of obesity in the West is the result of overeating and underexercising. Cigarettes are responsible for lung cancer. In addition, most of our present infectious diseases are related in some way to the rise of human civilization and the domestication of livestock and, though unwanted, of rodents. In the next chapter, we ponder the spread of disease and the changes in virulence, much of which can also be blamed on human activity.